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HAN ET AL.
2021
A Shelf Water Cascading Event near Cape Hatteras
LU HAN,a HARVEY SEIM,a JOHN BANE,a ROBERT E. TODD,b AND MIKE MUGLIAc
a
Department of Marine Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
b
Woods Hole Oceanographic Institution, Woods Hole, Massachusetts
c
Coastal Studies Institute, East Carolina University, Wanchese, North Carolina
(Manuscript received 18 July 2020, in final form 23 March 2021)
ABSTRACT: Carbon-rich Middle Atlantic Bight (MAB) and South Atlantic Bight (SAB) shelf waters typically converge
on the continental shelf near Cape Hatteras. Both are often exported to the adjacent open ocean in this region. During a
survey of the region in mid-January 2018, there was no sign of shelf water export at the surface. Instead, a subsurface layer of
shelf water with high chlorophyll and dissolved oxygen was observed at the edge of the Gulf Stream east of Cape Hatteras.
Strong cooling over the MAB and SAB shelves in early January led to shelf waters being denser than offshore surface
waters. Driven by the density gradient, the denser shelf waters cascaded beneath the Gulf Stream and were subsequently
entrained into the Gulf Stream, as they were advected northeastward. Underwater glider observations 80 km downstream of
the export location captured 0.44 Sv (1 Sv [ 106 m3 s21) of shelf waters transported along the edge of the Gulf Stream in
January 2018. In total, as much as 7 3 106 kg of carbon was exported from the continental shelf to a greater depth in the open
ocean during this 5-day-long cascading event. Earlier observations of near-bottom temperature and salinity at a depth of
230 m captured several multiday episodes of shelf water at a location that was otherwise dominated by Gulf Stream water,
indicating that the January 2018 cascading event was not unique. Cascading is an important, yet little-studied pathway of
carbon export and sequestration at Cape Hatteras.
KEYWORDS: Continental shelf/slope; Fronts; In situ oceanic observations
1. Introduction
Shelf–open ocean exchanges play important roles in the
global carbon budget (Walsh 1991; Bauer et al. 2013), the
transport of nutrients, pollutants, heat and biomass, as well as
the modulation of storm tracks and intensity, with significant
environmental, economic, and societal implications (Lentz
and Fewings 2012; Todd et al. 2019). Cape Hatteras, North
Carolina, the dividing point between the Middle Atlantic Bight
(MAB) and South Atlantic Bight (SAB) along the U.S. East
Coast, is an active region for shelf–open ocean exchanges because of the confluent western boundary currents and convergence of the adjacent shelf and slope waters (Verity et al.
2002; Jahnke 2010). It has long been recognized that a large
amount of carbon-rich shelf water is exported from the shelf
near Cape Hatteras (Fisher 1972; Churchill and Berger 1998;
Savidge and Bane 2001; Churchill and Gawarkiewicz 2012,
2014; Savidge and Savidge 2014). Exported shelf waters that
have been entrained along the northern edge of the Gulf
Stream have been observed hundreds of kilometers downstream of Cape Hatteras and are commonly referred to as
‘‘Ford Waters’’ (Church 1937; Ford et al. 1952; Kupferman and
Garfield 1977; Lillibridge et al. 1990). On the continental shelf,
an average of about 0.12 Sv (1 Sv [ 106 m3 s21) of warm and
saline SAB water (Savidge and Savidge 2014) and about
0.13 Sv of relatively cold and fresh MAB water flow toward and
Denotes content that is immediately available upon publication as open access.
Corresponding author: Lu Han, luhan@unc.edu
converge near Cape Hatteras (Churchill and Berger 1998;
Churchill and Gawarkiewicz 2012, 2014). The baroclinic front
between these two distinct water masses is known as the
‘‘Hatteras Front’’ (HF; Stefánsson et al. 1971; Pietrafesa et al.
1994; Savidge 2002). Export pathways for shelf water at the
front are highly variable and not yet well defined (Churchill
and Gawarkiewicz 2012; Savidge and Savidge 2014).
Dense shelf water cascading down the continental slope is an
important mechanism for shelf water export, leading to both
carbon export from the continental shelf and carbon sinking to
deeper layers in the open ocean where it is sequestered from
surface exchange (Stefánsson et al. 1971; Yoder and Ishimaru
1989; Shapiro and Hill 1997; Shapiro et al. 2003; Ivanov et al.
2004; Ulses et al. 2008; Sanchez-Vidal et al. 2009). However,
among an extensive inventory of observed cases of dense water
cascades globally (Ivanov et al. 2004), no such event was reported near Cape Hatteras, possibly due to the intermittency,
subsurface nature, and limited spatial extent of shelf water
cascades. The closest sites were on the SAB shelf along the
Georgia coast (Stefánsson et al. 1971; Yoder and Ishimaru
1989). However, subsurface MAB shelf waters have been observed being carried along the edge of the Gulf Stream (GS)
over 200 km downstream of Cape Hatteras as part of the Ford
Water (Churchill et al. 1989; Lillibridge et al. 1990). These
instances of subsurface shelf waters were observed below GS
surface waters, but after the GS detached from the continental
slope and thus have not been previously identified as cascading
events. However, they might have been first entrained below
the GS water by the cascading process near Cape Hatteras
and then advected downstream with the Gulf Stream.
The life cycle of a cascading event can be divided into several
stages (Gawarkiewicz and Chapman 1995; Shapiro et al. 2003;
DOI: 10.1175/JPO-D-20-0156.1
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FIG. 1. (a) Sea surface salinity along the ship track of Cruise AR-26, the locations of three
NDBC buoys (44014, 41025, 41064), four PEACH buoys/moorings (B1, B2, A4, A7), the
NCROEP mooring, and the Spray glider’s track during 13–27 Jan 2018. Exported shelf water
was observed along the magenta portion of the glider track. The red line marks the location of
repeated cross-shelf transects shown in Fig. 2. (b) SST image from 0912 UTC 19 Jan 2018, the
day on which cast 77 was taken.
Ivanov et al. 2004). The Gulf Stream has significant impact on
the complex dynamic processes near Cape Hatteras (Churchill
and Berger 1998; Savidge and Bane 2001; Savidge 2002;
Churchill and Gawarkiewicz 2014; Savidge and Savidge 2014).
We propose the following four-stage dynamic framework for
the cascading of shelf waters at the HF and subsequent entrainment by the Gulf Stream: 1) the preconditioning stage,
during which the shelf water becomes denser than the adjacent
GS water; 2) the falling stage, during which the shelf water
flows down the sloping bathymetry in the form of a gravity
current; 3) the turning stage, during which the shelf water
reaches a depth where it is neutrally buoyant within the GS and
turns left due to its entrainment into the Stream; and 4) the
final stage, during which the shelf water flows with the GS.
Here we document for the first time a shelf water cascading
event at the HF, which occurred during January 2018, with
detailed observations over and immediately adjacent to the
continental shelf. Together these observations reveal the characteristics and dynamics of shelf water cascading at the HF.
2. Data and methods
Most of the observations used in this study are from the
Processes Driving Exchange At Cape Hatteras (PEACH)
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FIG. 2. Temperature and salinity along the repeated cross-shelf transect at 35.758N on (a) 11 Jan, (b) 14 Jan, and
(c) 19 Jan 2018. Temperature contours (58, 108, 158, and 208C) are red, and salinity contours (34, 34.5, 35, and 36) are
black. The locations of the numbered CTD casts and B1 are shown along the upper axes. The black dots mark where
there were valid measurements.
program. The PEACH field program (April 2017–November
2018) utilized a wide range of observing platforms. During the
January 2018 cascading event, we have observations from a research vessel, a Spray glider, surface buoys, subsurface moorings, and satellite remote sensing (Fig. 1a) that complement each
other in time and space.
a. Shipboard data
During the second of three PEACH cruises aboard R/V Neil
Armstrong (Cruise AR-26), hydrographic casts throughout the
Hatteras region, underway ADCP data, and surface temperature and salinity measurements along the ship track (Fig. 1a)
were collected during 7–19 January 2018. Seventy-seven CTD
casts were collected using a multisensor equipped Sea-Bird
9111 CTD rosette. Water mass properties, including in situ
temperature, practical salinity, chlorophyll a, dissolved oxygen, and turbidity, were measured during each cast. All of
these parameters were processed with the SBE CTD data
processing software (Sea-Bird Electronics Inc. 2013) and the
Gibbs SeaWater (GSW) Oceanographic Toolbox of TEOS-10
(McDougall and Barker 2011). Chlorophyll concentration was
computed as a standard conversion from the voltage measured
directly by the fluorometer with the manufacturer’s calibration
curve, leading to slight negative values of the lowest chlorophyll
concentration. Our results and conclusions are not significantly
affected by this since we primarily use the chlorophyll information to infer relative differences between the shelf water and
the GS water. Three underway ADCPs operated throughout the
cruise: a Teledyne RDI (TRDI) Workhorse 300 kHz (WH300),
a TRDI Ocean Surveyor 150 kHz (OS150), and a TRDI Ocean
Surveyor 38 kHz (OS38), with vertical resolutions of 2, 5, and
20 m, respectively. Surface temperature and salinity along the
ship track were measured by the shipboard Sea-Bird thermosalinograph and were processed and calibrated on board.
During Cruise AR-26, a cross-shelf transect on the MAB shelf
at 35.758N was repeated three times (11, 14, and 19 January;
Figs. 1a and 2). TheÐ transport
of the MAB shelf water was calÐx
culated as TMAB 5 x21 y dx dz, where y is northward velocity
measured by the ship’s WH300 ADCP and the integration is
over the full water depth z and over the cross-shelf range [x1, x2]
where the surface salinity measured by the shipboard thermosalinograph was less than or equal to 34.5. This surface isohaline
is a common choice to delineate the MAB shelfbreak front
(Linder and Gawarkiewicz 1998). The transport calculation assumes vertically well mixed conditions, an assumption supported
by CTD casts along the transect (Fig. 2).
b. Glider observations
A Spray autonomous underwater glider (Sherman et al.
2001) collected profiles of temperature, salinity, absolute
currents (Todd et al. 2017), chlorophyll fluorescence, and
acoustic backscatter along and across the shelfbreak north of
Cape Hatteras and within the Slope Sea (Fig. 1a), between
35.58 and 36.98N during 16 November 2017–10 March 2018
(Todd 2020b). Following Todd (2020b), the shelf water
transport through a glider transect was calculated as
Tshelf water 5 ååy?,ij H(S0 2 Sij )Dxj Dzi ,
i
j
where y ?,ij is the flow locally perpendicular to the transect
(positive offshore), H is the Heaviside function, S0 is a reference
salinity, Sij is local salinity, Dxj is the distance covered during
each glider dive, and Dzi 5 10 m is the vertical bin size. Here we
use S0 5 35 rather than the S0 5 34.5 used by Todd (2020b)
because, as we show below, S0 5 35 bounds the exported shelf
observed in January 2018. Chlorophyll transport was defined as
Tchl 5 ååCChl,ij y ?,ij H(S0 2 Sij )Dxj Dzi ,
i
j
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where CChl,ij is the measured chlorophyll concentration. The
chlorophyll measurements from the glider are calibrated only by
applying the factory specified scaling to the measured voltage
and are then used for a rough estimation of the carbon export.
c. Buoy and mooring observations
We use meteorological data from three National Data Buoy
Center (NDBC) surface buoys (44014, 41025, and 41064) and
two PEACH surface buoys (B1 and B2; Fig. 1a). Wind speed
and direction were directly measured by the buoys. The net
atmosphere-to-ocean heat flux was computed from COARE
bulk flux algorithm, version 3.6 (Fairall et al. 2003; Edson
et al. 2013):
Qnet 5 SWnet 2 Qlat 2 Qsen 2 LWnet ,
where SWnet is shortwave radiation corrected for albedo into
the sea surface, Qlat is latent heat flux, Qsen is sensible heat flux,
and LWnet is net longwave radiation.
At the two PEACH buoys, temperature and salinity were
measured at 5 m depth with Sea-Bird CTDs. In addition, we
also use the temperature and depth averaged velocity data
from the bottom moored ADCPs at four PEACH shelf
moorings (B1, A4, A7, and B2) that were located roughly along
the 30-m isobath. We define the along-shelf direction to be the
orientation of the major principal axis of the depth-average
velocity. The major principal axis direction is roughly parallel
to the local isobath at each shelf location. The positive alongshelf direction is poleward.
d. Dissipation estimates
Under the assumption of the proportionality relationship
between the Ozmidov scale LO (Ozmidov 1965) and the
Thorpe scale LT, we estimate the dissipation rate for a particular CTD cast from the Thorpe scale LT (Thorpe 1977, 2005)
as 5 c1 L2T N 3 , where the constant c1 5 (LO/LT)2 5 0.64
(Dillon 1982) and N is buoyancy frequency. The Thorpe scale
is defined as the root-mean-square (rms) of the vertical displacements between the observed profile and the resorted
profile under stable stratification. Standard processing of each
CTD cast collected with the SBE 911 CTD results in a density
accuracy of 1023 kg m23 and vertical resolution of 1 m. Thus,
the minimum resolvable dissipation rate is 2 3 1028 W kg21.
Ship heave can significantly degrade performance; however,
for the particular cast of interest below, the sea conditions were
calm and there were few pressure loops in the raw data.
3. Results
Throughout the winter of 2017/18, glider observations
indicated a nearly complete cessation of MAB shelf water (S #
34.5) export from the continental shelf between the northern
edge of the Gulf Stream near 35.58N and the northern end of
the glider survey near 36.98N (Todd 2020b). Of 12 repeated
transects from December 2017 through February 2018, only a
single glider transect occupied during 13–19 January, examined
below, had seaward MAB shelf water transport close to
the long-term average (Todd 2020b). During the winter of
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2017/18, a filament of warm and relatively saline waters occupied the outer continental shelf and upper continental slope,
extending northward from the northern edge of the Gulf
Stream near 35.78N to near 388N (Fig. 1b; Todd 2020b). Both
glider- and ship-based observations indicated that this water
originated in the Gulf Stream and was less dense than the adjacent shelf water, leading to a reversal of the typical crossshelfbreak density gradient (Todd 2020b) in the MAB where
denser slope waters typically abut shelf waters (Gawarkiewicz
et al. 2018). This regional-scale setup in the winter of 2017/18
resembled the instances of discharged Gulf Stream water along
the edge of the continental shelf that were previously examined
by Churchill and Cornillon (1991).
During the AR-26 cruise, no surface signature of MAB shelf
water (S # 34.5 or T # 128C; Savidge et al. 2013) being exported to the adjacent open ocean was evident in either the
thermosalinograph data collected along the ship’s track or in
available sea surface temperature (SST) imagery (Fig. 1).
However, as much as 0.21 Sv of MAB shelf water flowed
southward toward Cape Hatteras over the southern MAB
shelf at 35.758N on 14 January (Figs. 1a and 2). Throughout the
duration of the cruise, no MAB water was observed at B2
(Fig. 3b), the southernmost PEACH mooring, indicating that
the MAB water was not flowing very far past Cape Hatteras
and onto the SAB shelf. It follows that MAB shelf water was
likely exported from the shelf between the latitude of B2
(34.788N) and the southern end of the glider transect near
35.78N, where the Gulf Stream flows very near to the edge of
the continental shelf.
a. Observations of a subsurface export event in
January 2018
The final CTD cast of the AR-26 cruise on 19 January,
hereinafter referred to as ‘‘cast 77,’’ captured a layer of relatively low salinity and low temperature water beneath a surface
layer of warm and salty GS water at the edge of the GS (Fig. 3).
This layer extended vertically for well over 100 m from a depth
of about 80 m to the bottom at 220 m with salinity ranging from
around 34 to 35 and temperature ranging from about 108 to
138C, clearly different from the surface GS water directly
above and from other GS profiles (Fig. 3). High chlorophyll
and dissolved oxygen in the same depth range of cast 77 are
characteristic of shelf water (Fig. 3f; Churchill and Gawarkiewicz
2012; Savidge and Savidge 2014). However, no water with these
properties was found in any other shipboard CTD casts or the
continuously measured thermosalinograph data (Fig. 3b).
Within the high chlorophyll layer, the freshest and coldest
point has a salinity of 34 and a temperature of 108C (herein we
refer to this as ‘‘34/10 water’’), which falls within the climatological T and S ranges of MAB water in January (Fig. 3b, blue
ellipse; Savidge et al. 2013). As is clear from Fig. 2, there was a
large change in the hydrography on the southern MAB shelf
during the cruise, and the MAB shelf water was constrained
inshore of the 40-m isobath. The northernmost PEACH
mooring, B1, was located at the front between the MAB shelf
water and the GS filament water on the outer MAB shelf,
where some 34/10 water appeared very briefly on 15 January
(Fig. 3b). That 34/10 water possibly formed as a result of mixing
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FIG. 3. (a) The locations of CTD casts taken during the AR-26 cruise. Different colors represent casts used in the water mass analysis.
(b) A T–S diagram of all of the CTD measurements, with colors as in (a). The gray dots are thermosalinograph data from the cruise. The
data at B1 and B2 are also during the cruise (11–19 Jan). (c) Chlorophyll and (d) dissolved oxygen vs salinity at selected shipboard CTD
casts, with colors as in (a). (e) Profiles of (e) temperature and salinity and (f) dissolved oxygen and chlorophyll, with cast 77 as solid lines
and profiles within the GS as dashed lines.
between the cold and fresh MAB shelf water and the warm and
salty GS water in the filament (Fig. 3b). The presence of waters
of MAB origin beneath Gulf Stream waters and seaward of the
shelf break suggests that these exported waters were subducted
as they left the continental shelf.
On the T–S curve for cast 77, mixing signatures are evident
from the freshest and coldest point (the 34/10 water) toward
higher and lower densities (Fig. 3b). The 34/10 water has the
highest values of chlorophyll and dissolved oxygen at cast 77.
The similar two-limb curves in T–S, chlorophyll–S, and dissolved oxygen–S spaces (Figs. 3b–d) suggest that the MAB
water is mixing with different water sources at the upper and
lower layers. The upper limb suggests mixing between the
34/10 water and the surface GS water. Such mixing may have
occurred as the shelf water subducted beneath the GS. The
lower limb, at depths between 100 and 220 m, likely indicates
mixing between shelf waters with differing properties, which
may have already existed before the subduction. In addition to
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FIG. 4. (a) The estimated dissipation rate (solid black line) and observed density profile at cast 77 (solid red line). The
thin dashed red lines are density at the Gulf Stream profiles, shown in Fig. 3a. (b) The current speed (dashed black lines)
and direction (dashed red lines) within a 1-km radius of cast 77, measured by the shipboard ADCP. The solid black line is
the speed of the mean current. The solid red line is the direction of the mean current counterclockwise from true east.
the two distinct water masses on the southern MAB shelf already mentioned, there is a third shelf water mass, the waters
on the northern SAB shelf. The GS filament water on the MAB
outer shelf shared very similar properties with the SAB water,
which was warm and salty but with high chlorophyll and dissolved oxygen (Fig. 3). This is supported by both the shipboard
CTD casts made on the SAB shelf and MAB outer shelf
(Figs. 3c,d), as well as the overlapped temperature and salinity
data at B1 and B2 (Fig. 3b). Since the typical GS position along
the SAB is immediately adjacent to the continental shelf, both
the circulation and hydrography on the outer shelf of the SAB
are often dominated by the GS (Atkinson et al. 1983; Castelao
et al. 2010). SAB water shares some hydrographic properties
with GS upper-layer water. However, the relatively long residence time of the water on the SAB shelf (9–11 weeks on average; Savidge and Savidge 2014) allows primary production to
increase the chlorophyll and dissolved oxygen concentrations
in the water. Likewise, the GS filament water on the MAB
outer shelf also had higher chlorophyll and dissolved oxygen
relative to the GS water. Accordingly, these two waters will be
both categorized as ‘‘modified Gulf Stream’’ (MGS) water. It is
possible that the lower limb was due to the mixing between the
MAB shelf water with either one of the MGS waters before the
subduction. Herein we refer to the exported shelf water as
‘‘shelf water mixture’’ (SWM).
Since there was no surface signature of shelf water export,
the SWM at cast 77 likely resulted from a subsurface shelf
water export event due to cascading. The SWM on the shelf
was laterally adjacent to the GS, but was more than 1 kg m23
denser than the GS mixed layer water. Such dense water sitting
at the top of the slope would be unstable and tend to flow down
the slope. The SWM would cascade down the continental slope
until the denser SWM reaches its own isopycnal along the flank
of the GS and is then entrained into the GS. It would take some
time and distance for the SWM to transform from a gravity
current flowing down the continental slope to become part
of the GS flow under an approximate geostrophic balance.
Propagation of bottom-trapped density driven currents can
produce intense turbulent mixing due to entrainment of ambient water and bottom friction (Price et al. 1993). At cast 77,
the mean turbulence dissipation rate of the SWM layer (below
100 m) was about 2.85 3 1026 W kg21, over 10 times that within
the surface mixed layer (upper 30 m; Fig. 4a), which is evidence
of active turbulent mixing (Thorpe 2005). Furthermore, the
speed of the SWM had much smaller vertical variation than
that of the upper layer and the direction of the depth-averaged
flow over the SWM layer was about 178 to the right of the
overlying GS flow (Fig. 4b). Taken together, these support
the hypothesis that the SWM measured at cast 77 was in the
turning stage, transforming from a gravity current flowing
down the continental slope and turning left to the poleward
direction due to the GS entrainment.
b. Observations of the final stage
Glider observations provide evidence of the final stage in the
cascade process in the form of a roughly 100-m-thick layer of
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FIG. 5. Spray glider observations along successive southbound (13–19 Jan) and northbound (19–26 Jan) transects
along the upper continental slope: (a) temperature, (b) salinity, (c) T–S diagram, (d) chlorophyll fluorescence,
(e) eastward velocity, and (f) northward velocity along the transect. The black contours are isopycnals, and the gray
contours are the 35 isohaline.
relatively cold and freshwater with high chlorophyll approximately 80 km downstream of cast 77 on 17–18 January (Fig. 5).
This is the only instance of shelf waters observed along the
upper continental slope by the glider from December 2017
through February 2018. This layer of water was bounded by the
35 isohaline, making it distinct from GS water, but it shared
similar properties with the SWM sampled at cast 77 (Figs. 5a–d).
The similar two-limb mixing feature of the exported shelf water
at both cast 77 and along the glider track suggests that the
water observed by the glider could have been produced during
the same event as the SWM at cast 77, or at least through the
same process of SWM cascading from the shelf. The gliderobserved layer of SWM lay between the 26 and 26.75 isopycnals, sloping toward the core of the GS. It was moving
with the GS at a speed of about 0.2–0.9 m s21 northeastward
(Figs. 5e,f). The total transport for the water with salinity less
than or equal to 35 through the glider transect in the Slope
Sea was 0.44 Sv, which is almost double the sum of the
mean alongshore transport on both the MAB and the SAB
shelves (Savidge and Savidge 2014). This implies that the SMW
entrained a large amount of the ambient water while cascading
along the continental slope, as observed in previous measurements of gravity currents (e.g., Price et al. 1993). No such water
was observed after the glider turned back to the north on
19 January, indicating the transient nature of the event (Fig. 5).
c. Time scale of the event
The temporal duration of the export event can be estimated
from the combined dataset. The mean velocity of the exported
SWM measured by the glider was 0.48 m s21. At that speed, it
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FIG. 6. (a) The net heat flux near Cape Hatteras. The red line is the hourly net heat flux
at 41025. The black line is the mean daily net heat flux of the five buoys near Cape
Hatteras. The shaded area is 1 standard deviation of the daily estimates. (b) Stick plot
(black) and magnitude (red) of wind stress at 41025. (c) The along-shelf velocity and
(d) bottom temperature at 4 PEACH moorings (B1, A4, A7, and B2, from north to south).
The vertical purple dashed lines mark the beginning and the end of the exported shelf
water from glider observation, and the vertical green dashed line marks the time at which
cast 77 was made.
would have taken the water about 3 days to move with the GS
from the cascading location near cast 77 to where it was encountered by the glider about 80 km downstream. In this case,
the glider-observed SWM should have been exported from the
shelf on 14–15 January. Southward winds began to increase on
14 January (Fig. 6b) and the waters on both the MAB and SAB
shelf off Cape Hatteras were pushed equatorward in the alongshelf direction (Fig. 6c). This southward movement lasted for
about 5 days, until 19 January, which was the longest southward shelf water flow episode off Cape Hatteras during the
entire winter of 2017/18 (Fig. 6c). The stronger equatorward
along-shelf flows and the much lower bottom temperature at
the moorings on the MAB shelf (B1 and A4) relative to the
ones on the SAB shelf together suggest that the HF stayed in
Raleigh Bay between moorings A4 and A7 within this 5-day
period (Figs. 1, 6c,d). It is conceivable that either one export
event lasted for 5 days or that two separate, but closely spaced
in time, episodes of shelf water export were measured, one by
the glider and another by cast 77.
d. Preconditioning by atmospheric cooling
The waters on the MAB and SAB shelves near Cape
Hatteras lost substantial heat to the atmosphere during the
2017/18 winter. The daily net heat flux measured by surface
buoys on the shelf had been from ocean to atmosphere for
almost 2 months before the export event (Fig. 6a). During that
time, shelf waters were losing heat to the atmosphere at an
average rate of 345 W m22 and with maximum heat loss of at
least 2217 W m22 during passage of a ‘‘bomb cyclone’’ in early
January (Hirata et al. 2019). If we neglect lateral advection and
consider only one-dimensional heat transfer, a 30-m-deep
column of seawater could cool by as much as 158C in 2 months,
assuming an average net heat flux of 345 W m22 from ocean to
atmosphere. For shelf waters with salinities of 34–35, cooling
from 208 to 58C would increase density by about 3 kg m23.
However, warmer SAB water and colder MAB water were
advected to the Cape Hatteras region. Based on our mooring
observations, the temperature of the MAB water decreased by
about 108C (at B1) and that of SAB water (at B2) decreased by
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FIG. 7. (a) The essential features and their relative positioning
during the shelf water cascading event. The black circle and white
triangle mark the locations of cast 77 and the NCROEP mooring,
respectively. (b) Schematic of the cascading event at the transect
along the Hatteras Front [white dashed line in (a)]; Q represents
spatially variable heat flux.
about 58C between mid-November and mid-January (Fig. 6d).
Considering a salinity increase of 1.5 at B1, the MAB water
accordingly became 3 kg m23 denser while the SAB water
density increased by 2 kg m23. During the cruise, the maximum
observed density of MAB shelf water was 1026.8 kg m23 while
SAB waters had a maximum density of 1026.7 kg m23 (Fig. 3b).
The GS surface layer had a density of 1025 kg m23 (Fig. 4a).
The excess density of the shelf waters due to the cooling preconditioned the system for the cascading event.
4. Discussion
The processes and driving dynamics of shelf water export
in the Cape Hatteras region are complex, with a wide range
of forcing variability (e.g., the GS, atmospheric forcing, shelf
water properties, and local bathymetry; Churchill and Berger
2029
1998; Savidge and Bane 2001; Gawarkiewicz and Linder 2006;
Churchill and Gawarkiewicz 2012; Savidge et al. 2013; Churchill
and Gawarkiewicz 2014; Savidge and Savidge 2014). The export
is largely event-driven (e.g., Todd 2020b). Examination of discrete shelf water export events, as we do here, allows for
studying the characteristics and dynamics of shelf water export
under certain combinations of forcing.
Using a combination of ship, buoy, mooring and glider observations, we have documented a shelf water cascading event
in which a 100-m thick layer of SWM was entrained beneath
the shoreward edge of the Gulf Stream (Figs. 3 and 5).
However, we are unable to conclusively determine the origin
of the SWM, since waters with the same properties were not
observed anywhere else on the shelf, except for the 34/10 tip
appearing at B1 briefly. We can, however, speculate about
potential formation mechanisms for SWM based on regional
hydrographic observations.
When the HF advanced southwestward past Cape Hatteras
into Raleigh Bay on 15 January, four different water types met
at the HF: 1) fresh and cold MAB shelf water, 2) GS water, 3)
the GS filament water, and 4) SAB shelf water (Fig. 1b).
These last two shared very similar properties and abutted
the MAB water at the eastern and southwestern sides, respectively (Fig. 1b). The SWM observed at cast 77 and by the
glider could have formed through mixing of MAB shelf
water with either the GS filament water on the MAB outer
shelf or the SAB water. The SWM could be part of the
frontal water between the MAB shelf water and the GS
filament water on the southern MAB mid and outer shelf,
advected to the HF with the MAB shelf water. It is also
plausible that the mixing was between the MAB and SAB
waters and occurred at the HF. Along the HF, the SWM
flowed to the boundary between the MAB shelf water and
the GS, where it cascaded.
We now present a hypothesized dynamic framework of the
shelf water cascading event in January 2018 (Fig. 7). We focus
on the cross-shore structure near the HF when it was south
of Cape Hatteras, since ship- and glider-based observations
showed no shelf water exported along the southern MAB shelf
edge during and prior to this event. In this framework, we divide the process into four stages.
1) Preconditioning stage: The extensive cooling on the MAB
and SAB shelves near Cape Hatteras in the 2017/18 winter
coincided with the preconditioning stage (section 3d). As a
result of the surface cooling, the density of the waters on
both the MAB and SAB shelf increased. At the time of the
cascade event in January 2018, the SWM at the HF front
was more than 1 kg m23 denser than the adjacent GS surface mixed layer water.
2) Falling stage: The negative buoyancy acquired from the
density gradient drove the SWM to cascade down the upper
continental slope in the form of a gravity current, until
reaching its neutral density along the edge of the GS
(Fig. 7). There it could intrude into the GS following the
sloping isopycnal. A 5-day time scale for the cascading
event (section 3c) falls within the typical GS meandering
periodicity of 3–8 days near Cape Hatteras (Savidge 2004).
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FIG. 8. Sea surface temperature (SST) imagery during five probable cascading events from (a)–(e) the earlier mooring observations and
(f) the January 2018 event, overlaid with the locations of cast 77 and the NCROEP mooring. The gray dots are all data from the bottom
CTD on the NCROEP mooring, and the red dots are those during each event. Black squares show observations from cast 77 for
comparison.
The passage of a GS meander can also play a role in the
falling process. The downward motion of the cascading
SWM can be enhanced by the usual downwelling within the
trailing portion of a GS meander’s frontal eddy (which is
along the leading edge of a GS meander crest). Similarly,
cascading could be impeded by upwelling within the leading
portion of a frontal eddy (at the trailing edge of the previous
meander crest; see Bane et al. 1981; Lee and Atkinson 1983;
Osgood et al. 1987; Gula et al. 2016).
3) Turning stage: As the SWM flowed down the continental
slope, it would tend to turn right (equatorward in this
location) because of the Coriolis effect; however, the large
onshore–offshore sea surface slope of the GS provides a
shoreward pressure gradient force, which could slow the
cascade’s downslope flow and thereby reduce its Coriolis
effect, allowing for a left-turning tendency. The velocity at
cast 77 suggests that the exported SWM was in the stage of
turning left to join the poleward-flowing GS (section 3a). As
part of the process of being entrained into the GS, the exported SWM would tend to turn left due to Ekman dynamics
from bottom drag (Ekman 1905), turbulent mixing with the
poleward-flowing GS water, and form drag from the GS.
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HAN ET AL.
4) Final stage: In the final cascading stage, the SWM reached
its neutral density, became part of the GS, and moved
downstream. The glider observations 80 km downstream
of cast 77 captured a roughly 100-m-thick layer of SWM
transported northeastward with the GS (section 3b).
Some instances of the subsurface shelf waters observed by
Churchill et al. (1989) and Lillibridge et al. (1990) might
have been in the final stage of the cascading process, when
the exported shelf waters were carried along the edge of
the GS over 200 km downstream of Cape Hatteras. The
temperature and salinity of these subsurface shelf waters
were not as low as 108C and 34, which might be due to the
seasonality of the shelf water properties or a combination of
double diffusion and shear-induced turbulence (Churchill
et al. 1989; Lillibridge et al. 1990).
To summarize, a variety of external forcing mechanisms,
including atmospheric cooling, wind, and interactions with
GS frontal eddies, can conspire to generate a cascading
event (Fig. 7b).
The cascading event at the cast 77 location in January 2018 is
not unique. Prior to the PEACH field program, a bottommoored ADCP/CTD was maintained from 2014 to 2017 on the
upper slope near Cape Hatteras in water depth of about 230 m
(Fig. 1a) by the North Carolina Renewable Ocean Energy
Program (NCROEP; General Assembly of North Carolina
2010; Muglia 2019). The location of the NCROEP mooring was
4.2 km from the location of cast 77 (Figs. 1 and 7a). Nearbottom salinity and temperature observations at the NCROEP
mooring site usually indicated the presence of GS waters,
but the record was punctuated by multiple instances of cool,
fresh MAB shelf water along the seafloor (Fig. 8, inset T–S
diagrams). These episodes usually occurred during winter
and lasted for several days each (Muglia 2019). During each
of these episodes, the SST pattern was very similar to that
during the cascading event in January 2018 (Fig. 8). The HF
was to the south of Cape Hatteras, and the seaward flank of
the HF was the boundary between the MAB shelf water and
the leading portion of a Gulf Stream meander crest (trailing
portion of a frontal eddy; Fig. 7a). The large temperature
gradient between the shelf water and the Gulf Stream suggests strong horizontal convergence, which we believe is a
signature of denser shelf water being subducted under the
lighter Gulf Stream water, as was seen during the January
2018 event. The multiyear observations from the NCROEP
mooring suggest that cascading of shelf waters at this location
may occur during most winter/spring seasons, and that conditions favoring subduction of shelf water at this location may
be part of the typical seasonal cycle.
This type of cascading may be an important pathway for
carbon export near Cape Hatteras due to the large amount of
carbon carried by the chlorophyll-rich shelf waters (Yoder and
Ishimaru 1989; Churchill and Gawarkiewicz 2014; Savidge and
Savidge 2014). The mean chlorophyll concentration in the
cascaded SWM at cast 77 was more than 1 mg m23 and that
measured by the glider about 80 km downstream was about
0.7 mg m23 (Fig. 5d). About 0.3 kg s21 of chlorophyll was carried by the 0.44-Sv SWM flow across the glider transect.
2031
Assuming a carbon to chlorophyll ratio of 50 (Yoder and
Ishimaru 1989) and steady export, roughly 7 3 106 kg of carbon
would have been exported from the shelf to the open ocean
during a 5-day-long cascading event. From a numerical simulation, Fennel and Wilkin (2009) estimated annually averaged carbon export across the MAB shelfbreak of about 50 3
106 mol C yr21 per kilometer of shelf. The estimated carbon
export along the entire length of the MAB shelf over 5 days is
nearly equivalent to that during the 5-day cascading event. As
with other export events documented in the MAB (e.g.,
Cenedese et al. 2013; Chen et al. 2014; Todd 2020b), this
short-duration event can dominate the annually averaged
export, though we note that carbon export near Cape
Hatteras may be greater than the MAB-wide average inferred by Fennel and Wilkin (2009) due to the mean alongshelf convergence. Unlike the surface pathways, cascading
cannot only export this large amount of carbon in the shelf
water offshore to the open ocean, but it can transport the
carbon to depths of 200 m or more, beneath the GS surface
mixed layer.
5. Conclusions
Carbon-rich MAB and SAB shelf waters converge at the
HF off Cape Hatteras, where they can be exported to
the open ocean. Episodic shelf water cascading events
in the winter constitute one of several different export
pathways for MAB and SAB shelf waters near Cape
Hatteras. We have used extensive observations collected
during the PEACH field program to document a cascading
event that occurred in January 2018. We observed 0.44 Sv
of SWM transport at the edge of the GS, after being exported from the continental shelf. The estimated total
carbon export during the event was 7 3 10 6 kg, which is
comparable to the total carbon export along the entire
length of the MAB shelf over 5 days. We divide the cascading event into four stages:
1) During the preconditioning stage, SWM became denser
than the adjacent GS mixed layer water due to sufficient
heat loss to the atmosphere. The seaward flank of the HF
became the front between the SWM and the GS, due to
the HF being pushed to the south of Cape Hatteras by
northerly winds.
2) In the falling stage, this large density gradient drove the
denser SWM to subduct beneath the GS water and cascade
down the continental slope as a gravity current. The falling
stage could have been modulated by GS meandering and
vertical motions associated with a passing GS frontal eddy.
3) During the turning stage, the SWM turned left while being
entrained by the GS.
4) Last, the SWM flowed downstream as a part of the GS. A
number of other likely cascading events observed earlier at
the NCROEP mooring indicate that this is a recurring
process and not a unique event.
The cascading process could play an important role in the local
carbon budget by rapidly transporting carbon-rich shelf water
beneath the surface layer in the open ocean.
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JOURNAL OF PHYSICAL OCEANOGRAPHY
Acknowledgments. This research was funded by the National
Science Foundation (Grants OCE-1558920 to University of
North Carolina at Chapel Hill and OCE-1558521 to Woods
Hole Oceanographic Institution) as part of PEACH. We
acknowledge and thank Sara Haines for the processing and
QC of the mooring data, and we thank the PEACH group
for helpful discussions and for their support. Additional
thanks are given to the crew of R/V Armstrong (AR-26). We
also acknowledge North Carolina Renewable Ocean Energy
Program for providing the CTD data at the NCROEP
mooring. We thank two anonymous reviewers for their input
and suggestions.
Data availability statement. The cruise data are available
online (https://www.rvdata.us/search/cruise/AR26). Sea surface temperature (SST) was measured by NOAA’s Advanced
Very High Resolution Radiometer (AVHRR), with spatial
resolution of 1 km and was obtained from the Mid-Atlantic
Regional Association Coastal Ocean Observing System
(MARACOOS) THREDDS server (http://tds.maracoos.org/
thredds/ARCHIVE-SST.html). Spray glider observations from
PEACH are available online (http://spraydata.ucsd.edu) and
should be cited using https://doi.org/10.21238/S8SPRAY0880
(Todd 2020a). Details of the CTD data at the NCROEP
mooring and how to request access are available from mugliam@
ecu.edu at East Carolina University Coastal Studies Institute.
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